Antibodies to a malignant melanoma cell-derived antigen were detected in sera from normal tumor free persons as well as in sera from patients with melanoma. In another study antibodies to the line-10 guinea pig hepatocarcinoma were found in sera from 20 normal unimmunized strain-2 guinea pigs. Both malignant melanoma and line-10 tumor cells have antigenic determinants in common with BCG and other bacteria and antibodies in normal sera may have been induced by bacterial stimulation. BCG-SS, a soluble extract of BCG, was found to induce and elicit antibodies and delayed cutaneous hypersensitivity in rabbits, guinea pigs, and humans. It was also shown to be an adjuvant and mitogen. It increased the cytotoxicity of effector cells in antibody dependent cytotoxicity reactions and could sensitize lymphocytes from normal persons in vitro so that they became cytotoxic to human malignant melanoma and breast tumor cells. The feasibility of using Protein A-containing Staphylococcus aureus to precipitate antigen-antibody complexes in order to measure antibodies in sera from several mammalian species was studied. It was found to be a reliable method but certain antigens bound directly to S. aureus and all classes of human immunoglobulins tested, in particular IgG3 and IgA1 were not precipitated by S. aureus.